In the manufacture of such substrates, it has become commonplace to produce printed circuitry on both sides of the rigid or flexible substrate, in addition to within the substrate as mentioned above. Of increased importance is the manufacture of multilayered substrates, and particularly printed circuit boards (PCB's), the latter being the most common types of substrates sold today in view of increased operational demands for such products. In these products, the substrate typically consists of parallel, planar, alternating innerlayers of insulating substrate (also referred to as dielectric) material and conductive metal. Known such dielectric materials include fiberglass-reinforced epoxy resin (also referred to in the field as, simply, “FR4”), polytetrafluoroethylene (e.g., Teflon, a trademark of E.I. DuPont deNemours & Company), Driclad material (a trademark of Endicott Interconnect Technologies, Inc., the assignee of this invention), etc. The exposed outer sides of the laminated structure are often provided with circuit patterns as with double-sided boards, and the metal inner layers typically contain circuit patterns, except in the case of internal power planes which are substantially solid, albeit also containing clearance openings or other openings if desired.
In double-sided and multilayered circuitized substrates, it is necessary to provide conductive interconnections between the various conductive layers or sides of the product. This is commonly achieved by providing metallized, conductive thru-holes in the board which communicate with the opposite sides and selected layers requiring electrical interconnection. For some applications, it is desired that electrical connection be made with almost if not all of the conductive layers. In such a case, conductive thru-holes are provided through the entire thickness of the board. For these, as well as other applications, it is often desired to also provide electrical connection between the circuitry on one face of the board and only one or more of the inner circuit layers. In those cases, “blind vias”, passing only part way through the board, are provided. In still another case, such multilayered boards often require internal “vias” which are located entirely within the board's structure and covered by external layering, including both dielectric and conductive. Such internal “vias” are typically formed within a sub-part structure of the final board and then combined with other layers during final lamination of the board. For purposes of this application, the terms “thru-hole” is meant to include conductive thru-holes that pass entirely through the board (also referred to in the printed circuit board field as plated-thru-holes or PTHs), “blind vias” which extend from an external surface of the board into a specified conductive layer of the board, as well as “internal vias” which are “captured” by the board's outer layers.
To provide the desired circuit pattern(s) for the board, the art has developed a variety of manufacturing processes, many of which fall into the broad categories of “subtractive” or “additive” techniques. Common to subtractive processes is the need to etch away (or subtract) metal to expose substrate surface in areas where no circuitry is desired. Additive processes, on the other hand, begin with exposed substrate surfaces (or thin commoning metallization layers for additive electroplate) and build up thereon of metallization in desired areas, the desired areas being those not masked by a previously-applied pattern of plating resist material (e.g., called photo-resist material in the printed circuit board field). Both of said processes involve what is referred to as photolithographic processing in which desired patterns are defined using such photo-resist, which is exposed and “developed: to subsequently allow etching away of undesired metallurgy. Typically, the individual conductive layers are “personalized” using such photolithographic processing with the desired circuit patterns thereon and then laminated together with other such conductive layers with the dielectric layers located there-between. Lamination is a well-known PCB manufacturing process involving elevated temperatures and pressures and further description is not considered essential here. If outer circuit patterns are desired, the substrate includes outer conductive layers which are similarly “personalized”.
Typically, thru-holes are drilled (including mechanically or more recently using lasers) or punched into or through the board at desired locations. In the case of internal vias, these are formed within the desired individual layers prior to lamination. In the case of blind vias, these are formed from a desired outer surface, usually after final lamination. In the case of PTH's, these are preferably formed from a desired outer surface of the completely laminated structure and extend entirely through the structure to the opposite side. Once such holes are initially provided (drilled or punched), the hole walls (of the dielectric material in the substrate) may then be metallized, generally by utilization of electro-less metal depositing techniques, albeit other deposition processes are also known in the field. Such a process results in one or more thin layers of conductor (usually copper) on the internal walls of the holes, this/these conductive layer(s) in turn being electrically coupled to the selected internal and external conductive layers.
It is also known with respect to the formation of conductive thru-holes in circuitized substrates such as PCB's and chip carriers to utilize conductive paste within the openings, either in combination with the already plated conductor surfaces or simply as the only conductive medium for the length of the thru-hole. Several known examples of such conductive pastes exist, one known example being sold under the product name “TH9910” by Tatsuta System Electronics Company, of Osaka, Japan. This paste includes a copper powder filler and a phenolic resin binder. Pastes including silver powders are also known, one example being sold under the product name “ED2000” by Electra, of Tonbridge, England. As defined herein, the present invention provides a new and unique method and apparatus for providing such deposition in a manner readily adaptable to mass production.
Examples of methods of making PCB's, including providing same with such thru holes, are shown and described in the following U.S. Letters Patents:    U.S. Pat. No. 6,015,520 Method For Filling Holes in Printed Wiring Boards    U.S. Pat. No. 6,493,861 Interconnected Series of Plated Through Hole Vias and Method of Fabrication Therefor    U.S. Pat. No. 6,630,630 Multilayer Printed Wiring Board and Its Manufacturing Method    U.S. Pat. No. 6,630,743 Copper Plated PTH Barrels and Methods For Fabricating    U.S. Pat. No. 6,631,558 Blind Via Laser Drilling System    U.S. Pat. No. 6,631,838 Method For Fabricating Printed Circuit Board    U.S. Pat. No. 6,638,690 Method For Producing Multi-Layer Circuits    U.S. Pat. No. 6,638,858 Hole Metal-Filling Method    U.S. Pat. No. 6,809,269 Circuitized Substrate Assembly And Method of Making Same    U.S. Pat. No. 6,828,514 High Speed Circuit Board And Method Of Making    U.S. Pat. No. 6,905,589 Circuitized Substrate And Method of Making Same
As stated, the present invention defines a new and unique process for depositing conductive paste within circuitized substrate openings to produce a final substrate having the beneficial aspects thereof. The following U.S. Letters Patents describe various processes and apparatus for depositing pastes:    U.S. Pat. No. 5,133,120 Method of Filling Conductive Material Into Through Holes of Printed Wiring Board    U.S. Pat. No. 5,191,709 Method of Forming Through-Holes In Printed Wiring Board    U.S. Pat. No. 5,287,806 Apparatus and System For Screen Printing of Solder Paste Onto Printed Circuit Boards    U.S. Pat. No. 5,925,187 Apparatus For Dispensing Flowable Material    U.S. Pat. No. 6,272,984 Squeegee For Screen Printing and Screen Printing Method    U.S. Pat. No. 6,286,422 Method and Apparatus For Dispensing Viscous Material    U.S. Pat. No. 6,840,169 Methods For Manufacturing Printed Circuit Boards Using a Partial Printing Process and Apparatus
The present invention provides a method and apparatus for accomplishing conductive paste deposition within circuitized substrates in a manner considered more expedient than those mentioned above, both of which are considered readily adapted for mass production. It is believed that such a method and an apparatus for accomplishing said method will represent significant advancements in the art.